PROJECT SUMMARY Obsessive-compulsive disorder (OCD) is one of the most disabling, chronic psychiatric disorders, with a lifetime prevalence of 2-3%. Emerging findings point to a significant role for basal ganglia circuits in OCD. Despite this, our understanding of the molecular pathophysiology of OCD remains inadequate, and our treatment options leave most patients with continued impairment. The best-replicated genetic finding in OCD is association with SLC1A1, encoding the neuronal glutamate, aspartate, and cysteine transporter EAAT3/EAAC1. However, the impact of this gene on the normal and abnormal functioning of OCD-related circuits is unknown. To fill this knowledge gap, we developed a STOP-TetO knock-in mouse line that allows us to flexibly manipulate Slc1a1 expression. Using dopamine agonists as a probe, we found that EAAT3 loss decreases basal ganglia-mediated repetitive, stereotyped behavior. Our convergent data support the hypothesis that increased EAAT3 function plays a role in OCD pathology and that decreasing EAAT3 activity may serve as a novel treatment option. Little is known, however, about EAAT3's molecular and functional impact in the basal ganglia. Elsewhere in the brain, EAAT3-mediated transport decreases neurotransmission at perisynaptic glutamate receptors and provides substrate for GABA and glutathione synthesis, but it is unclear which of these functions is important in basal ganglia circuits, and whether EAAT3's impact on dopaminergic neurotransmission is pre- or post-synaptic. Using our flexible mouse model and previously established OCD optogenetic and transgenic mouse models, this R01 will 1) examine effects of EAAT3 ablation and targeted rescue on basal ganglia function and repetitive behavior, and 2) determine if EAAT3 ablation leads to symptom resolution in phenotypically-similar but etiologically-independent mouse models of OCD with abnormal basal ganglia signaling. These data could be leveraged to demonstrate a clear treatment target that motivates development of promising EAAT3 inhibitor lead compounds.